Methods of forming a phase change material

ABSTRACT

Methods of forming a phase change material are disclosed. The method includes forming a chalcogenide compound on a substrate and simultaneously applying a bias voltage to the substrate to alter the stoichiometry of the chalcogenide compound. In another embodiment, the method includes positioning a substrate and a deposition target having a first stoichiometry in a deposition chamber. A plasma is generated in the deposition chamber to form a phase change material on the substrate. The phase change material has a stoichiometry similar to the first stoichiometry. A bias voltage is applied to the substrate to convert the stoichiometry of the phase change material to a second stoichiometry. A phase change material, a phase change random access memory device, and a semiconductor structure are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/008,173, filed Jan. 18, 2011, pending, which is a divisional of U.S.patent application Ser. No. 12/191,446, filed Aug. 14, 2008, now U.S.Pat. No. 7,888,165, issued Feb. 15, 2011, which application is relatedto U.S. patent application Ser. No. 12/204,510, filed Sep. 4, 2008,entitled “A PHASE CHANGE MATERIAL, A PHASE CHANGE RANDOM ACCESS MEMORYDEVICE INCLUDING THE PHASE CHANGE MATERIAL, A SEMICONDUCTOR STRUCTUREINCLUDING THE PHASE CHANGE MATERIAL, AND METHODS OF FORMING THE PHASECHANGE MATERIAL,” now U.S. Pat. No. 7,834,342, issued Nov. 16, 2010.This application is also related to U.S. patent application Ser. No.12/909,665, filed Oct. 21, 2010, entitled “PHASE CHANGE MEMORY DEVICESAND METHODS OF FORMING A PHASE CHANGE MATERIAL,” now U.S. Pat. No.8,124,956, issued Feb. 28, 2012, and to U.S. patent application Ser. No.13/347,919, filed Jan. 11, 2012, pending, entitled “METHODS OF FORMING APHASE CHANGE MATERIAL.” The disclosure of each of the above-identifiedapplications and patents is hereby incorporated herein by this referencein its entirety.

TECHNICAL FIELD

Embodiments of the present invention relate to methods of producing aphase change material having a desired stoichiometry. More specifically,the present invention, in various embodiments, relates to producing aphase change material having a stoichiometry that is different from thestoichiometry of a target used in its formation, a heterogeneous phasechange material, and structures incorporating the phase change material.

BACKGROUND

Phase change materials are known in the art and include compounds formedfrom germanium (Ge), antimony (Sb), and tellurium (Te), which are knownas GST materials. The phase change material is capable of beingreversibly electrically switched between an amorphous state and acrystalline state. The phase change material is electrically writableand erasable and has been used in electronic memory applications. Whenthe GST material is in the amorphous state, it is said to be “reset,”while the GST material is said to be “set” in the crystalline state. GSTmaterials have been used in phase change random access memory (“PCRAM”)devices to provide non-volatile memory with long data retention. PCRAMdevices rely on the electrically bistable status of resistancedifferences between the amorphous and crystalline states of the GSTmaterial.

One GST material used in PCRAM devices is Ge₂Sb₂Te₅. However, duringoperation of the PCRAM device, changes in the stoichiometry of the GSTmaterial have been observed. In other words, the GST material, asdeposited, includes different relative amounts of Ge, Sb, and Te thanthe GST material after operation of the PCRAM device. In addition, thestoichiometry of the Ge, Sb, and Te has been reported to change in anactive region or contact region of the PCRAM device after repeatedoperation. While the relative amount of Ge in the Ge₂Sb₂Te₅ in theactive region remained constant, the Ge₂Sb₂Te₅ became Sb-rich andTe-deficient. However, regions of the Ge₂Sb₂Te₅ not subject to theswitching maintained their original stoichiometry.

It would be desirable to form a phase change material having a desiredstoichiometry, where the stoichiometry differs from that of a depositiontarget used to form the phase change material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic illustration of a deposition system in accordancewith an embodiment of the invention;

FIG. 2 is a schematic illustration of a heterogeneous phase changematerial formed in accordance with an embodiment of the invention;

FIGS. 3-13 are cross-sectional views illustrating the fabrication of aPCRAM device in accordance with an embodiment of the invention;

FIG. 14 is a graph illustrating the effect of increasing bias voltageapplied to a substrate on the stoichiometry of a GST material formed onthe substrate; and

FIGS. 15 and 16 are graphs illustrating the effect of no bias voltage(FIG. 15) versus a bias voltage of 250 W (FIG. 16) applied to thesubstrate on the stoichiometry of a GST material formed on thesubstrate.

DETAILED DESCRIPTION

A method of forming a phase change material having a desiredstoichiometry or ratio of elements is disclosed. As used herein, thephrase “phase change material” means and includes a chalcogenidecompound formed from a chalcogen ion and at least one electropositiveelement. By applying a bias voltage to a substrate, upon which the phasechange material is deposited, as the phase change material is deposited,the stoichiometry of the elements of the phase change material may becontrolled or adjusted. As such, the phase change material having thedesired stoichiometry is produced. As used herein, the term “biasvoltage” means and includes a fixed or pulsed DC voltage applied to thesubstrate through a chuck or support. The bias voltage to be applied tothe substrate may be achieved by setting a bias voltage at a specifiedcurrent, setting a bias current at a specified voltage, setting a biaspower as a combination of voltage and current, or combinations thereof.For convenience, the bias voltage may be described herein as beingapplied to the substrate, when in actuality, the bias voltage is appliedto the substrate through the chuck. The bias voltage refers to thevoltage measured between the substrate and a plasma. Application of thebias voltage during deposition of the phase change material may causesputtering of at least a portion of at least one element of the phasechange material, resulting in the phase change material having a reducedamount of that element. Since the stoichiometry of the elements in thephase change material is controlled as the phase change material isdeposited, a single deposition target may be used to form phase changematerials having different stoichiometries. In addition, the method maybe used to form a phase change material having a heterogeneous or asubstantially homogeneous composition throughout its thickness.

The following description provides specific details, such as materialtypes, material thicknesses, and processing conditions in order toprovide a thorough description of embodiments of the present invention.However, a person of ordinary skill in the art will understand that theembodiments of the present invention may be practiced without employingthese specific details. Indeed, the embodiments of the present inventionmay be practiced in conjunction with conventional fabrication techniquesemployed in the industry. In addition, the description provided hereindoes not form a complete process flow for manufacturing a PCRAM device,and the PCRAM device described below does not form a completesemiconductor device. Only those process acts and structures necessaryto understand the embodiments of the present invention are described indetail below. Additional acts to form a complete semiconductor deviceincluding the PCRAM device may be performed by conventional techniques.

The illustrations presented herein are not meant to be actual views ofany particular systems, phase change materials, or PCRAM devices, butare merely idealized representations that are employed to describeembodiments of the present invention. Elements and features commonbetween figures may retain the same numerical designation.

The chalcogen ion of the phase change material may be oxygen (O), sulfur(S), selenium (Se), Te, or polonium (Po). In one embodiment, thechalcogen ion is Te. The electropositive element may include, but is notlimited to, nitrogen (N), silicon (Si), nickel (Ni), gallium (Ga), Ge,arsenic (As), silver (Ag), indium (In), tin (Sn), Sb, gold (Au), lead(Pb), bismuth (Bi), or combinations thereof. In one embodiment, theelectropositive elements are Ge and Sb. The chalcogenide compound may bea binary, ternary, or quaternary alloy of these elements. By way ofnon-limiting example, the chalcogenide compound may be a compound of Ge,Sb, and Te (a GST material). The GST material may have an empiricalformula of Ge_(x)Sb_(100−(x+y))Te_(y), where the stoichiometry (inatomic percent) of Ge and Te are indicated by x and y, respectively, andthe remainder of the GST material is Sb. By way of non-limiting example,x may be greater than approximately 5 atomic percent but less thanapproximately 60 atomic percent, such as between approximately 17 atomicpercent and approximately 44 atomic percent, and y may be greater thanapproximately 20 atomic percent but less than approximately 70 atomicpercent, such as between approximately 23 atomic percent andapproximately 56 atomic percent. By way of non-limiting example, the GSTmaterial may be Ge₂₂Sb₂₂Te₅₅ (also known as Ge₂Sb₂Te₅), Ge₈Sb₃₂Te₅₆(also known as Ge₁Sb₄Te₇), Ge₁₄Sb₂₈Te₅₆ (also known as Ge₁Sb₂Te₄),Ge₄₀Sb₉Te₅₁, Ge₄₄Sb₅Te₅₁, Ge₂₈Sb₂₇Te₄₅, Ge₅₈Sb₁₉Te₂₃, Ge₁₇Sb₂₇Te₅₆,Ge₃₀Sb₁₇Te₅₃, or combinations thereof. While compounds having specificstoichiometries are listed above, the phase change material may includeother stoichiometries of Ge, Sb, and Te.

While specific examples herein describe the phase change material as aGST material, the phase change material may be a chalcogenide compoundformed from other elements. By way of non-limiting example, thechalcogenide compound may be a compound of Sb and Te, such as Sb₂Te₃, acompound of Ge and Te, such as GeTe, a compound of In and Se, such asIn₂Se₃, a compound of Sn and Te, such as SnTe, a compound of Bi and Te,such as Bi₂Te₃, a compound of Sb and Te, such as SbTe, a compound of Snand Se, such as SnSe, a compound of Ge and Se, such as GeSe, a compoundof Au, Ge, Sn, and Te, such as Au₂₅Ge₄Sn₁₁Te₆₀, a compound of Ag and Se,such as Ag₂Se, or a compound of In and Te, such as InTe. Whilechalcogenide compounds having specific stoichiometries are listed above,the chalcogenide compound may include the same combination of elementshaving other stoichiometries.

The substrate upon which the phase change material is formed comprises aconventional silicon substrate or other bulk substrate including a layerof semiconductor material. As used herein, the term “bulk substrate”includes not only silicon wafers, but also silicon-on-insulator (“SOI”)substrates, silicon-on-sapphire (“SOS”) substrates, epitaxial layers ofsilicon on a base semiconductor foundation, and other semiconductor oroptoelectronics materials, such as silicon-germanium, germanium, galliumarsenide, or indium phosphide. The material of the substrate may bedoped or undoped. The phase change material may also be formed onanother material overlying the substrate, depending on the intendedapplication for the phase change material. By way of non-limitingexample, if the phase change material is to be used in a PCRAM device,the phase change material may be formed on a titanium nitride (TiN),titanium aluminum nitride (TiAlN), or tungsten (W) material overlyingthe substrate.

To achieve the desired stoichiometry of the phase change material on thesubstrate, the phase change material may be deposited by a depositiontechnique in which a plasma is capable of being formed and a biasvoltage is capable of being applied to the substrate. By way ofnon-limiting example, the deposition technique may be a physical vapordeposition (“PVD”) technique or a chemical vapor deposition (“CVD”)technique. PVD includes, but is not limited to, sputtering, evaporation,or ionized PVD. Such deposition techniques are known in the art and,therefore, are not described in detail herein. However, other depositiontechniques in which a plasma is capable of being formed and a biasvoltage is capable of being applied to the substrate may also be used,such as pulsed laser deposition (“PLD”). Alternatively, the phase changematerial may be formed by another conventional deposition technique,followed by subsequent generation and application of the plasma and thebias voltage.

A system 100 for forming the phase change material 102 on the substrate104 is illustrated in FIG. 1. The substrate 104 may be positioned orplaced on a support or chuck (not shown) of a deposition chamber 106 inwhich the plasma 108 is capable of being produced and the bias voltageis capable of being applied to the substrate 104. The deposition chamber106 may be configured to produce the plasma 108 and apply the biasvoltage to the substrate 104 during deposition of the phase changematerial. The plasma 108 produced in the deposition chamber may be aninert plasma produced from a noble gas element, such as a helium, neon,argon, krypton, xenon, or radon. In one embodiment, an argon plasma isgenerated. As described in more detail below, the plasma 108 may alsoinclude nitrogen. The deposition chamber 106 may be configured forapplying a bias voltage of up to approximately 500 W to the substrate104 through the chuck. By way of non-limiting example, the depositionchamber 106 may be a conventional PVD chamber or PVD tool. Sinceconventional PVD chambers are capable of producing the plasma andapplying the bias voltage to the chuck, a conventional PVD chamber maybe used in the present invention without substantial modificationthereto. The deposition chamber 106 may also be configured forcontrolling the temperature of the chuck, as explained below. In oneembodiment, the deposition chamber is an Entron system, which iscommercially available from Ulvac Technologies, Inc. (Methuen, Ma.).

The deposition chamber 106 may also include a deposition target 110formed from a chalcogenide material having the same, or substantiallysimilar, combination of elements as those of the desired phase changematerial 102. The deposition target 110 may be selected by a person ofordinary skill in the art depending on the phase change material 102 tobe formed. By way of non-limiting example, the deposition target 110 maybe a Ge₂Sb₂Te₅ target, known as a 225 target, or a Ge₁Sb₄Te₇ target,known as a 147 target. Such deposition targets 110 are commerciallyavailable, such as from Nikko Materials USA, Inc. (Chandler, Ariz.), MMCTechnology, Inc. (San Jose, Calif.), and Umicore Group (Brussels,Belgium). In one embodiment, the deposition target 110 is a 225 target.

After positioning the substrate 104 on the chuck, the plasma 108 may begenerated in the deposition chamber 106 and the bias voltage may,simultaneously, be applied to the substrate 104. The conditions, such astemperature and pressure, for generating and maintaining the plasma 108in the deposition chamber 106 are conventional and, therefore, are notdescribed in detail herein. The bias voltage applied to the substrate104 may be up to approximately 500 W, such as from approximately 25 W toapproximately 200 W. As the deposition target 110 is bombarded with theplasma 108, atoms of the deposition target 110 are sputtered from thetarget surface and deposited on a surface of the substrate 104, forminga coating of the phase change material 102 on the surface of thesubstrate 104. The phase change material, as initially deposited, mayhave approximately the same stoichiometry as that of the depositiontarget 110. In other words, the phase change material coating thesurface of the substrate 104 may, initially and momentarily, haveapproximately the same stoichiometry of elements as that of thedeposition target 110. However, by applying the bias voltage as thephase change material 102 is deposited, a portion of the chalcogen inthe phase change material 102 may be sputtered, resulting in the phasechange material 102 having a reduced content of the chalcogen comparedto the chalcogen content in the as-deposited, phase change material. Forclarity and convenience, the phase change material as initiallydeposited is not illustrated. As such, the stoichiometry of theresulting phase change material 102 may be different than thestoichiometry of the deposition target 110.

The phase change material 102 on the substrate 104 may be formed in anamorphous state or in a crystalline state by adjusting the chucktemperature. If the chuck temperature is maintained at approximatelyroom temperature during the deposition of the phase change material 102,the phase change material 102 may be deposited in an amorphous state. Ata deposition temperature above room temperature, the phase changematerial 102 may be deposited in a crystalline state. By way ofnon-limiting example, the phase change material 102 is deposited incrystalline state. Alternatively, a portion of the phase change material102 may be deposited in the amorphous state and another portion of thephase change material 102 may be deposited in the crystalline state.

Without being bound by any theory, it is believed that applying the biasvoltage to the substrate 104 during deposition of the phase changematerial 102 may pull ions generated by the plasma 108, such as argonions, toward the substrate 104. As the plasma ions are acceleratedtoward the as-deposited material, the plasma ions may collide with theas-deposited phase change material 102. Contact between the plasma ionsand the as-deposited phase change material 102 may cause the individualelements of the as-deposited phase change material 102 to redistributeor be sputtered away. By way of non-limiting example, as the phasechange material 102 is being deposited, the plasma 108 may accelerateinto the as-deposited phase change material 102, causing atoms of theas-deposited phase change material 102, such as the chalcogen atoms, tobe sputtered away. Therefore, applying the bias voltage to the substrate104 may remove the chalcogen atoms from the as-deposited phase changematerial 102, producing a phase change material 102 having a reducedamount of the chalcogen relative to the amount of the chalcogen in theas-deposited phase change material 102. By increasing the bias voltageapplied to the substrate 104, the chalcogen content of the phase changematerial 102 may decrease.

By forming the phase change material 102 having the reduced amount ofchalcogen, the resistance of the phase change material 102, and theoverall resistance of a device in which the phase change material 102 ispresent, may be reduced. Accordingly, by tailoring the amount ofchalcogen in the phase change material 102, the resistance of the phasechange material 102 may be tailored. Furthermore, the decreasedchalcogen content may enable more consistent switching of the phasechange material 102.

In one embodiment, the phase change material 102 is a GST materialhaving the general empirical formula of: Ge_(x)Sb_(100−(x+y))Te_(y),where x and y are as previously defined. By way of non-limiting example,if the deposition target 110 is a Ge₂Sb₂Te₅ deposition target, theas-deposited phase change material 102 is, at least originally,Ge₂Sb₂Te₅. However, application of the bias voltage to the substrate 104causes Te atoms to be sputtered away, leading to a decreased Te contentin the phase change material 102. Changing the amount of Te in the GSTmaterial changes the electrical resistance of the GST material and theoverall resistance of a device in which the GST material is used.Consequently, by tailoring the amount of Te in the GST material, theresistance of the GST material may be tailored.

The GST material may be deposited in a crystalline state by maintainingthe temperature in the deposition chamber at above room temperature. Inone embodiment, the as-deposited phase change material 102 is acrystalline GST material since the resistance of the crystalline GSTmaterial is on the order of kiloOhms (kΩ), while the resistance of theamorphous GST material is on the order of megaOhms (MΩ).

The bias voltage applied to the substrate 104 may be held constant orvaried to form a substantially homogenous phase change material 102 or asubstantially heterogeneous phase change material 102. The phase changematerial 102 formed on the substrate 104 may be substantiallyhomogeneous in that the stoichiometry of the phase change material 102is constant, or of a single stoichiometry, throughout its thickness. Thesubstantially homogeneous phase change material 102 may be formed byapplying a constant bias voltage to the substrate 104 as the phasechange material 102 is deposited. By way of non-limiting example, if abias voltage of approximately 95 W is applied to the substrate 104 whiledepositing a GST material by PVD using a 225 deposition target, a GSTmaterial having approximately 49 atomic percent Te, approximately 24atomic percent Ge, and approximately 26 atomic percent Sb is produced.In contrast, if a bias voltage of 0 W is applied, a GST material havingapproximately 57 atomic percent Te, approximately 20 atomic percent Ge,and approximately 22 atomic percent Sb is produced.

Alternatively, the stoichiometry of the phase change material 102 formedon the substrate 104 may be substantially heterogeneous. The phasechange material 102 may be substantially heterogeneous in that the phasechange material 102 may include a stepwise change in stoichiometrythroughout its thickness or may include a substantially continuousgradient or change in stoichiometry throughout its thickness. Asillustrated in FIG. 2, the heterogeneous phase change material 102 maybe viewed as being formed of a plurality of portions 112, 112′, 112″,112′″, 112″″ (i.e., 112-112″″) (indicated using dashed lines), eachportion of the plurality of portions 112-112″″ having a differentstoichiometry. While FIG. 2 illustrates that the heterogeneous phasechange material 102 includes five portions 112, 112′, 112″, 112′″,112″″, the heterogeneous phase change material 102 may include fewer ormore portions 112-112″″. By way of non-limiting example, a first portion112 of the phase change material 102 may have a first stoichiometry, asecond portion 112′ of the phase change material 102 may have a secondstoichiometry, a third portion 112′ of the phase change material 102 mayhave a third stoichiometry, a fourth portion 112′″ of the phase changematerial 102 may have a fourth stoichiometry, and a fifth portion 112″″of the phase change material 102 may have a fifth stoichiometry. For thephase change material 102 to be heterogeneous, at least one of theportions 112-112″″ of the phase change material 102 may have the samestoichiometry as another of the portions 112-112″″ as long as at leastone of the portions 112-112″″ has a different stoichiometry than anotherof the portions 112-112″″. The plurality of portions 112-112″″ of thephase change material 102 may be substantially indistinguishable fromone another by visual detection. However, the differences instoichiometry may be detected by conventional spectroscopy orspectrometry techniques.

The heterogeneous phase change material 102 having a stepwise change instoichiometry may be formed by making stepwise changes to the biasvoltage applied to the substrate 104 during deposition of the phasechange material 102. By way of non-limiting example, if the phase changematerial 102 is to be a bilayer composition (i.e., include twostoichiometries), two bias voltages may be applied to the substrate 104in a stepwise manner. A first bias voltage may be applied to thesubstrate 104 and maintained for a desired amount of time, followed byapplying a second bias voltage to the substrate 104 and maintaining thesecond bias voltage for a certain amount of time. The second biasvoltage may be increased or decreased compared to the first biasvoltage, depending on the desired stoichiometries in the resulting phasechange material 102. To form a phase change material 102 having three ormore different stoichiometries, additional bias voltages may be appliedto the substrate 104 in a stepwise manner as the phase change material102 is deposited.

The heterogeneous phase change material 102 having a substantiallycontinuous gradient in the stoichiometry of the respective elements maybe formed by making a substantially continuous change to the biasvoltage applied to the substrate 104 during deposition of the phasechange material 102. By way of non-limiting example, if a substantiallycontinuous gradient of the phase change material 102 is desired, thebias voltage may be increased or decreased at a substantially constantrate during deposition of the phase change material 102.

Since the stoichiometry of the phase change material 102 may be selectedby controlling the bias voltage applied to the substrate 104, a singledeposition target 110 may be used to achieve phase change materials 102having different stoichiometries. In other words, phase change materials102 of differing stoichiometry may be produced from a single depositiontarget 110. Previously, using a conventional process to form a phasechange material having a desired stoichiometry, a deposition targethaving a specific, corresponding desired stoichiometry had to be used.As a consequence of this process limitation, different depositiontargets had to be purchased to form different phase change materials.

By forming a phase change material 102 having a reduced chalcogencontent, a PCRAM device including the phase change material 102 mayexhibit improved initial switching and may be operated withoutsubstantial conditioning of the device before use. In addition, thephase change material 102 may improve the reliability of the PCRAMdevice, leading to more consistent switching and greater enduranceduring the lifetime of the PCRAM device.

The method of producing the phase change material 102 having a reducedchalcogen content may also be utilized with a phase change material 102formed on the substrate 104 by atomic layer deposition (“ALD”). Afterforming the phase change material 102 from the chalcogenide compound byALD, the substrate 104 having the deposited phase change material 102may be transferred from an ALD chamber to the deposition chamber 106.The plasma 108 may be generated in the deposition chamber 106 and thebias voltage applied to the substrate 104, as previously described, toproduce the phase change material 102 having the reduced chalcogencontent.

A phase change material 102 including nitrogen therein may also beformed by the above-mentioned method. To form the nitrogen-containingphase change material, the plasma 108 to which the substrate 104 issubjected may include nitrogen (“N₂”) in combination with the noble gaselement. By way of non-limiting example, the substrate 104 may besubjected to a plasma 108 including argon and N₂. The bias voltage maybe applied to the substrate 104, as previously described, forming thenitrogen-containing phase change material having the reduced chalcogencontent. Including nitrogen in the phase change material 102 may improvethe switching of a device having the phase change material 102 byreducing the current in the device.

The phase change material 102 may be used in a PCRAM device 200, asillustrated in FIG. 3. While specific examples herein describe andillustrate the phase change material 102 in the PCRAM device 200, thephase change material 102 may be utilized in other PCRAM structures orin a complementary metal-oxide semiconductor (“CMOS”) device. The PCRAMdevice 200 includes a memory matrix or array (not shown) that includes aplurality of memory cells for storing data. The memory matrix is coupledto periphery circuitry (not shown) by a plurality of control lines. Theperiphery circuitry may include circuitry for addressing the memorycells contained within the memory matrix, along with circuitry forstoring data in and retrieving data from the memory cells. The peripherycircuitry may also include other circuitry used for controlling orotherwise ensuring the proper functioning of the PCRAM device 200.

The memory matrix includes a plurality of memory cells that are arrangedin generally perpendicular rows and columns. The memory cells in eachrow are coupled together by a respective word line (not shown), and thememory cells in each column are coupled together by a respective digitline 206. Each memory cell includes a word line node that is coupled toa respective word line, and each memory cell includes a digit line nodethat is coupled to a respective digit line 206. The word lines and digitlines 206 are collectively referred to as address lines. These addresslines are electrically coupled to the periphery circuitry so that eachof the memory cells can be accessed for the storage and retrieval ofinformation. The memory cell includes a memory element, such as aprogrammable resistive element, which is coupled to an access device(not shown), such as a diode. The memory element is formed from thephase change material 102. The diode may be a conventional diode, azener diode, or an avalanche diode, depending upon whether the diodearray of the memory matrix is operated in a forward biased mode or areverse biased mode. The memory element is coupled to the word line, andthe access device is coupled to the digit line 206. However, connectionsof the memory element may be reversed without adversely affecting theoperation of the memory matrix.

As shown in FIG. 3, the PCRAM device 200 includes substrate 104, digitline 206, n-doped polysilicon material 208, p-doped polysilicon material210, dielectric material 212, lower electrode 214, phase change material102, upper electrode 218, insulative material 220, oxide material 222,and contact hole 224 (filled with conductive material 225). The PCRAMdevice 200 may be formed by conventional techniques. By way ofnon-limiting example and as illustrated in FIG. 4, the digit lines 206may be formed in or on the substrate 104. By way of non-limitingexample, the digit line 206 may formed in the substrate 104 as a dopedN⁺ type trench. Access device 226 may be formed on top of the digit line206. The access device 226 may be a diode, or other device, formed bythe n-doped polysilicon material 208 and the p-doped polysiliconmaterial 210. Next, the dielectric material 212 may be formed on top ofthe p-doped polysilicon material 210. The dielectric material 212 may beformed from a suitable insulative or dielectric material, such as plasmaenhanced CVD (“PECVD”) SiO_(z), where z is 1 or 2, PECVD siliconnitride, or standard thermal CVD Si₃N₄.

A hard mask 228 may be deposited on top of the dielectric material 212and patterned to form an opening 230, as illustrated in FIG. 5. A spacermaterial 232 may be deposited over the hard mask 228 in a conformalfashion so that the upper surface of the spacer material 232 is recessedwhere the spacer material 232 covers the opening 230, as illustrated inFIG. 6. By way of non-limiting example, a dielectric material, such asCVD amorphous or polycrystalline silicon, may be used as the spacermaterial 232. The spacer material 232 may be anisotropically etchedusing a suitable etchant, such as HBr+Cl₂. The rate and time of the etchare controlled so that the spacer material 232 may be substantiallyremoved from the upper surface of the hard mask 228 and from a portionof the upper surface of the dielectric material 212 within the opening230, leaving sidewall spacers 232′ within the opening 230.

Once the sidewall spacers 232′ have been formed, an etchant may be usedto form a pore 234 in the dielectric material 212, as illustrated inFIG. 7. The etchant may be an anisotropic etchant that selectivelyremoves the dielectric material 212 bounded by the sidewall spacers 232′until the p-doped polysilicon material 210 is reached. The hard mask 228and the sidewall spacers 232′ may be removed, as illustrated in FIG. 8,such as by etching or by chemical mechanical planarization (“CMP”). Thepore 234 may be filled to a desired level with a material suitable toform the lower electrode 214, as illustrated in FIG. 9. The lowerelectrode 214 may be formed using collimated PVD or another suitabledirectional deposition technique such that the lower electrode 214 isformed on top of the dielectric material 212 and within the pore 234.The lower electrode 214 on top of the dielectric material 212 may beremoved, using CMP, for example, to leave the lower electrode 214 at thebottom of the pore 234, as illustrated in FIG. 10. The lower electrode214 may be formed from at least one material, and may be formed in atleast one layer or other three-dimensional configuration. For instance,a layer of carbon may be used as a barrier material to prevent unwantedmigration between the subsequently deposited phase change material 102and the p-doped polysilicon material 210. A layer of titanium nitride(TiN) may then be deposited upon the layer of carbon to complete theformation of the lower electrode 214. Additional materials that may beused to form the lower electrode 214 include, but are not limited to,TiAlN or W.

The phase change material 102 may be deposited so that the phase changematerial 102 contacts the lower electrode 214, as illustrated in FIG.11. A thickness at which the phase change material 102 is deposited maydepend on the size of the lower electrode 214. By way of non-limitingexample, if the lower electrode 214 is circular and has a diameter ofapproximately 40 nm, the phase change material 102 may be deposited at athickness of from approximately 400 Å to approximately 2000 Å. The phasechange material 102 may be a substantially homogeneous material or aheterogeneous material, as previously described. The upper electrode 218may be deposited on top of the phase change material 102, as illustratedin FIG. 12. The upper electrode 218 may be formed from TiN or othersuitable material. After the upper electrode 218, the phase changematerial 102, the dielectric material 212, and the access device havebeen patterned and etched to form an individual memory cell, theinsulative material 220, such as silicon nitride, is deposited over thestructure, as illustrated in FIG. 13. The oxide material 222 may then bedeposited over the insulative material 220. The oxide material 222 maybe patterned and the contact hole 224 formed through the oxide material222 and the insulative material 220. The contact hole 224 may then befilled with a conductive material 225 to form the word line and producethe PCRAM device 200 shown in FIG. 3.

At least a portion of the phase change material 102 may be capable ofbeing reversibly electrically switched between a first state and asecond state, where the first state and the second state differ in atleast one property that is detectable including, but not limited to,electrical resistivity, electrical conductivity, optical transmissivity,optical absorption, optical refraction, optical reflectivity,morphology, surface topography, relative degree of order, relativedegree of disorder, or combinations thereof. By way of non-limitingexample, the phase change material 102 may be configured to electricallyswitch between an amorphous state and a crystalline state, between afirst amorphous state and a second amorphous state, or between a firstcrystalline state and a second crystalline state, where the first andsecond states have at least one of the different detectable propertiesmentioned above, such as different resistivities. As used herein, thephrase “amorphous state” refers to a state in which the phase changematerial 102 has a less ordered, or more disordered structure orarrangement of atoms, while the phrase “crystalline state” means andincludes a state in which the phase change material 102 has a moreordered, or less disordered structure or arrangement of atoms. The phasechange material 102 may be switched between the first and second statesin a time period of approximately a few nanoseconds with the input ofpicojoules of energy. By way of non-limiting example, the phase changematerial 102 may be switched from the amorphous state to the crystallinestate in an amount of time ranging from approximately 50 nsec toapproximately 500 nsec. The phase change material 102 may be switchedfrom the crystalline state to the amorphous state in an amount of timeranging from approximately 5 nsec to approximately 100 nsec. The phasechange material 102 may be switchable between the first and secondstates for a sufficient number of times without exhibiting substantialchanges in at least one of the detectable properties mentioned above. Inone embodiment, the phase change material 102 is the GST material and isswitched between an amorphous GST material and a crystalline GSTmaterial.

The PCRAM device 200 may utilize a high current pulse to switch thephase change material 102 to the first state and a low current pulse toswitch the phase change material 102 to the crystalline state. By way ofnon-limiting example, if the phase change material 102 is the GSTmaterial, the high current pulse may switch the GST material to theamorphous state, while the low current pulse may switch the GST materialto the crystalline state. The GST material may be electrically switchedbetween the amorphous state and the crystalline state at a high switchrate or high switch speed and a low energy level.

In use and operation, a voltage is applied between the word line and thedigit line 206 of the PCRAM device 200. The current is applied to heatup a contact region 236 (FIG. 3) or active region between the phasechange material 102 and the lower electrode 214. During the high currentpulse, the phase change material 102 is subject to a temperature aboveits melting point, causing at least a portion of the phase changematerial 102 to convert from its crystalline state to the amorphousstate. The portion of the phase change material 102 converted from thecrystalline state to the amorphous state is indicated in FIG. 3 withdashed lines and corresponds to the contact region 236. The phase changematerial 102 remains in the amorphous state until the low current pulseis applied of sufficient duration to convert the phase change material102 to the crystalline state. A current density of from approximately1×10⁵ amperes/cm² to approximately 1×10⁷ amperes/cm² is used to switchthe phase change material 102 between the amorphous and crystallinestates in the contact region 236. To obtain this current density in acommercially viable PCRAM device 200 having at least 64 million memorycells, for instance, the contact region 236 of each memory cell is madeas small as possible to minimize the total current drawn by the PCRAMdevice 200. Heat may be generated in the contact region 236 where thephase change material 102 contacts the lower electrode 214 due to acurrent supplied through the lower electrode 214. The heat generated bythe current may convert the state of the phase change material 102 fromamorphous to crystalline.

A SET state of the PCRAM device 200 may be achieved by applying thevoltage or current pulse sufficient to raise the temperature of thephase change material 102 in the contact region 236 to below its meltingpoint but above its crystallization temperature. The temperature of thephase change material 102 may be maintained for a sufficient amount oftime to enable the atoms to be rearranged into a crystalline state. ARESET state of the PCRAM device 200 may be achieved by applying avoltage or current pulse sufficient to raise the temperature of thephase change material 102 in the contact region 236 to its meltingpoint. The temperature is maintained for a shorter time than the SETpulse. The SET pulse is typically longer in duration but of loweramplitude than the RESET pulse. The RESET pulse is typically shorter induration but of higher amplitude than the SET pulse. The actualamplitudes and durations of the pulses depend upon the size of thememory cells and the particular phase change material 102 used in thememory cells. RESET currents for phase change materials 102 used inmemory cells typically range from approximately 400 microAmpere (μA) toapproximately 600 μA and have durations of from approximately 10nanoseconds to approximately 50 nanoseconds, whereas SET currents rangefrom approximately 100 μA to approximately 200 μA and have durations offrom 50 nanoseconds to approximately 100 nanoseconds.

The phase change material 102 is non-volatile and may maintain theintegrity of the information stored in the PCRAM device 200 without theneed for periodic refresh signals, and the data integrity of the storedinformation is not lost when power to the PCRAM device 200 is removed.The phase change material 102 may be directly overwritable so that thePCRAM device 200 need not be erased in order to change informationstored within the PCRAM device 200. The phase change material 102 may beelectrically switched between the amorphous state and the crystallinestate at a high switch rate or switch speed and a low energy level.

The following examples serve to explain embodiments of the presentinvention in more detail. These examples are not to be construed asbeing exhaustive or exclusive as to the scope of this invention.

EXAMPLES Example 1

GST materials were deposited by PVD on six silicon oxide substrateswhile a constant bias voltage of approximately 0 W, approximately 15 W,approximately 25 W, approximately 45 W, approximately 65 W, orapproximately 90 W was applied to the silicon oxide substrate. The PVDdeposition was conducted using an argon plasma generated in a DentonDiscovery 24 Sputtering system and a Ge₂Sb₂Te₅ deposition target. ThePVD deposition chamber was maintained at 10 mTorr, and 150 W RF powerwas applied to the Ge₂Sb₂Te₅ deposition target during the deposition.The relative amounts of Ge, Sb, and Te were measured by inductivelycoupled plasma (“ICP”) spectrometry.

As shown in FIG. 14, the GST material deposited at 0 W had approximately57 atomic percent Te, approximately 20 atomic percent Ge, andapproximately 22 atomic percent Sb. At approximately 15 W, the GSTmaterial had approximately 55 atomic percent Te, approximately 21 atomicpercent Ge, and approximately 23 atomic percent Sb. At approximately 25W, the GST material had approximately 55 atomic percent Te,approximately 21 atomic percent Ge, and approximately 23 atomic percentSb. At approximately 45 W, the GST material had approximately 53 atomicpercent Te, approximately 22 atomic percent Ge, and approximately 24atomic percent Sb. At approximately 65 W, the GST material hadapproximately 52 atomic percent Te, approximately 23 atomic percent Ge,and approximately 25 atomic percent Sb. At approximately 95 W, the GSTmaterial had approximately 49 atomic percent Te, approximately 24 atomicpercent Ge, and approximately 26 atomic percent Sb. As shown in FIG. 14,at higher bias voltages compared to lower bias voltages, the amount ofTe present in the GST material was reduced. However, the relativeamounts of Sb and Ge were substantially unchanged regardless of the biasvoltage.

Example 2

GST materials were deposited by PVD on silicon oxide substrates while aconstant bias voltage of 0 W or of 250 W was applied to the siliconoxide substrate. The PVD deposition was conducted using an argon plasmagenerated in an Entron system and a Ge₂Sb₂Te₅ deposition target. The PVDdeposition chamber was maintained at 3 mTorr, and 1 KW RF power wasapplied to the Ge₂Sb₂Te₅ deposition target during the deposition. Therelative amounts of Ge, Sb, Te, nitrogen (N), oxygen (O), and silicon(Si) were measured by X-ray photoelectron spectroscopy (“XPS”).

As shown in FIG. 15, the GST material formed when no bias voltage wasapplied to the silicon oxide substrate during the PVD depositionincluded approximately 50 atomic percent Te, as measured by XPS. Incontrast and as shown in FIG. 16, the GST material formed when a biasvoltage of 250 W was applied to the silicon oxide substrate during thePVD deposition included approximately 34 atomic percent Te, as measuredby XPS.

The relative amounts of Ge, Sb, and Te were also measured by ICP. TheGST material formed when no bias voltage was applied included 23.5atomic percent Ge, 22.7 atomic percent Sb, and 53.8 atomic percent Te.The resistivity of this GST material was 51.1 mohm-cm. The GST materialformed when a bias voltage of 250 W was applied included 28.4 atomicpercent Ge, 27.1 atomic percent Sb, and 44.6 atomic percent Te. Theresistivity of this GST material was 14.4 mohm-cm.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the inventionencompasses all modifications, variations and alternatives fallingwithin the scope of the invention as defined by the following appendedclaims and their legal equivalents.

1. A method of forming a phase change material, the method comprising:introducing a substrate to a chamber in which a deposition target isdisposed, the deposition target comprising a chalcogenide material; andsubjecting the substrate to a bias voltage while exposing the substrateto a plasma in the chamber to form on the substrate another chalcogenidematerial having a stoichiometry differing from a stoichiometry of thechalcogenide material of the deposition target.
 2. The method of claim1, wherein introducing a substrate to a chamber in which a depositiontarget is disposed comprises introducing the substrate to the chamber inwhich a deposition target comprising Ge₂Sb₂Te₅ or Ge₁Sb₄Te₇ is disposed.3. The method of claim 1, wherein subjecting the substrate to a biasvoltage while exposing the substrate to a plasma in the chambercomprises forming on the substrate another chalcogenide material havinga lower atomic percent of a chalcogen ion than the atomic percent of thechalcogen ion in the chalcogenide material of the deposition target. 4.The method of claim 3, wherein subjecting the substrate to the biasvoltage while exposing the substrate to the plasma in the chambercomprises forming on the substrate another chalcogenide material havingless tellurium than the chalocogenide material of the deposition target.5. The method of claim 1, wherein subjecting the substrate to a biasvoltage while exposing the substrate to a plasma in the chambercomprises subjecting the substrate to a constant bias voltage whileexposing the substrate to the plasma in the chamber.
 6. The method ofclaim 1, wherein subjecting the substrate to a bias voltage whileexposing the substrate to a plasma in the chamber comprises subjectingthe substrate to a step-wise bias voltage while exposing the substrateto the plasma in the chamber.
 7. The method of claim 1, whereinsubjecting the substrate to a bias voltage while exposing the substrateto a plasma in the chamber comprises applying the bias voltage to achuck supporting the substrate while exposing the substrate to theplasma in the chamber.
 8. The method of claim 1, wherein subjecting thesubstrate to a bias voltage while exposing the substrate to a plasma inthe chamber comprises subjecting the substrate to the bias voltage whileexposing the substrate to a plasma comprising at least one of helium,argon, krypton, xenon, and radon.
 9. The method of claim 1, whereinsubjecting the substrate to a bias voltage while exposing the substrateto a plasma in the chamber comprises subjecting the substrate to thebias voltage while exposing the substrate to a plasma comprisingnitrogen.
 10. A method of forming a phase change material, the methodcomprising: forming a region of a phase change material using adeposition target; and forming another region of the phase changematerial using the deposition target, the region of the phase changematerial and the another region of the phase change material havingdifferent stoichiometries of elements of the phase change material. 11.The method of claim 10, wherein forming a region of a phase changematerial using a deposition target comprises depositing a chalcogenidematerial on a substrate using the deposition target as a source for thechalcogenide material.
 12. The method of claim 11, wherein depositing achalcogenide material on a substrate comprises depositing by physicalvapor deposition the chalcogenide material on the substrate.
 13. Themethod of claim 10, wherein forming a region of a phase change materialusing a deposition target comprises forming the region of the phasechange material on a substrate using the deposition target, the regionof the phase change material and the deposition target having differentatomic percentages of a chalcogen ion.
 14. The method of claim 10,wherein forming a region of a phase change material using a depositiontarget comprises: depositing on a substrate an amount of a depositionmaterial having upon deposition a stoichiometry of elements matchingthat of a chalcogenide material of the deposition target; and applying abias voltage to the substrate to alter the stoichiometry of the amountof the deposition material to form the region of the phase changematerial.
 15. The method of claim 14, wherein forming another region ofthe phase change material using the deposition target comprises:depositing on the region of the phase change material another amount ofthe deposition material having upon deposition the stoichiometry ofelements matching that of the chalcogenide material of the depositiontarget; and applying an additional bias voltage to the substrate toalter the stoichiometry of the another amount of the deposition materialto form the another region of the phase change material on the region ofthe phase change material.
 16. A method of forming a phase changematerial, the method comprising: forming a region of a chalcogenidematerial on a substrate while subjecting the substrate to a biasvoltage; and altering the bias voltage to form another region of thechalcogenide material on the substrate, the region of the chalcogenidematerial and the another region of the chalcogenide material havingdifferent stoichiometries.
 17. The method of claim 16, wherein forming aregion of a chalcogenide material on a substrate while subjecting thesubstrate to a bias voltage comprises forming the region of achalcogenide material having an empirical formula ofGe_(x)Sb_(100−(x+y))Te_(y), wherein x ranges from approximately 5 atomicpercent to approximately 60 atomic percent and y ranges fromapproximately 20 atomic percent to approximately 70 atomic percent. 18.The method of claim 16, wherein forming a region of a chalcogenidematerial on a substrate while subjecting the substrate to a bias voltagecomprises forming a chalcogenide compound comprising a chalcogen ionselected from the group consisting of oxygen, sulfur, selenium,tellurium, and polonium and at least one electropositive elementselected from the group consisting of nitrogen, silicon, nickel,gallium, germanium, arsenic, silver, indium, tin, antimony, gold, lead,and bismuth.
 19. The method of claim 16, wherein forming a region of achalcogenide material on a substrate while subjecting the substrate to abias voltage comprises forming the region of the chalcogenide materialon the substrate while simultaneously subjecting the substrate to thebias voltage and a plasma.
 20. The method of claim 16, wherein forming aregion of a chalcogenide material on a substrate while subjecting thesubstrate to a bias voltage comprises forming the region of thechalcogenide material on the substrate while subjecting the substrate toa bias voltage of from approximately 25 W to approximately 200 W.